Viscous hydrocarbons, such as the Athabasca bitumen in Alberta, Canada, are challenging to recover from their subterranean formations. One successful recovery technique is Steam-Assisted Gravity Drainage ("SAGD"). Introduced in U.S. Pat. No. 4,344,485 to Butler, and described fully in the textbook Thermal Recovery of Oil and Bitumen, by Roger M. Butler, and published in 1991 by Prentice-Hall, Inc., SAGD is a thermal process for mobilizing viscous oils. Briefly, steam is injected from an upper well. Hydraulic communication is established between the upper well and a lower, horizontally extending production well. The steam forms a steam chamber. At the boundaries of the chamber, the steam condenses and heats the viscous oil, lowering its viscosity. The heated fluid (oil and condensed steam) drains downwardly, under the force of gravity, to the lower well. The heated fluid is produced from the lower well and is recovered at the surface.
The production of heated fluid is maintained on "steam trap control" such that the temperature of the fluid in the lower well must be maintained below the saturated steam temperature at that location. This ensures that steam doesn't break through to the oil-producing lower well.
If the steam chamber is operated at a sufficiently high pressure, the fluid flows naturally to the surface. This is called natural lift. Otherwise, if assistance is needed to get the fluid to the surface, artificial lift can be employed. Conventional artificial lift techniques include the use of pumps or gas lift, whereby gas is added to the fluid within the lower part of the well, at an elevation close to the heel of the horizontal well.
Artificial lift has often been especially problematic in thermal projects. If the operating pressure in the steam chamber is low relative to the depth of the well, gas lift may not be adequate. Lift pumps are disadvantaged due to high temperatures, the high fluid rates, the need for `steam trap control`, and because the water in the produced fluid readily flashes to steam during low pressure pump cycles, significantly reducing the pumping operating efficiency. One method of reducing flashing of steam is to use vertical production wells having sumps. A sump permits placement of the pump below the elevation of the formation. The hydrostatic head in the sump is correspondingly increased such that the heated fluid is considerably below its saturated steam condition when pumped, ensuring reasonable efficiencies.
Where the use of sumps is difficult or impractical, such as with SAGD having horizontal production wells, some of the water in the produced fluid flashes to steam inside the pump and the efficiency of the pump is drastically reduced. Flashing is further worsened because friction causes the fluid's pressure to drop along the horizontal well, approaching the heel portion where the pump would be located. This frictional pressure drop combined with heat transfer effects within the well may cause the fluid to be at saturated steam conditions prior to reaching the pump.
The SAGD process has been very successful in testing performed at an underground test facility ("UTF") located in the Athabasca oil sands in Northern Alberta. Fortunately, the formation at the UTF permits high enough pressures to be used to avoid the use of artificial lift. Other SAGD projects, such as those in the Peace River oil sand deposit, also in northern Alberta, need the assistance of and have successfully applied gas lift to achieve flow to surface.
In the largest oil sand deposit, the Athabasca oil sands, the oil-bearing payzone is frequently shallow or has gas or water sand thief zones which require the steam chamber pressure to be too low to provide adequate lift to the surface with standard gas lift. Flashing of water to steam, the elevated temperatures involved, and the high production flow rates effectively preclude the use of pumps.
Thus, providing an enhanced lift method capable of operation in these circumstances is an important addition to SAGD technology.